The present invention relates to a light scanning device and an image forming apparatus. And in particular, the present invention relates to a light scanning device having a polarization splitting device that splits a light beam having different polarization directions, and an image forming apparatus having the light scanning device.
For image recording using electrophotography, an image forming apparatus using a laser is widely used. In this case, the image forming apparatus has a light scanning device, and in a general image forming method the image forming apparatus uses a deflector (for example, a polygon mirror), scans a drum having photosensitivity in an axis direction of the drum by a laser beam, rotates the drum, and forms a latent image on a surface of the drum.
In recent years, in the image forming apparatus, to achieve advanced colorization and speeding up, a tandem-method image forming apparatus having a plurality of drums (normally, four drums) with photosensitivity (photosensitive bodies) has been widely used.
In the case of the tandem-method image forming apparatus, in order to perform optical writing on a plurality of photosensitive bodies by a writing unit used in the light scanning device, the number of light sources of the light scanning device is increased (for example, in the case of four photosensitive bodies, normally four light sources are needed.). Therefore, problems such as an increase in the number of components, an increase in cost, and so on occur.
There is an example of a light scanning device used in the tandem-method image forming apparatus does not require an increased number of light sources (see Japanese patent application publication number 2002-23085). In Japanese patent application publication number 2002-23085, a pyramidal mirror, or a plate mirror is used, and a light beam from a common light source scans different scanned faces. In this method, the number of light sources can be reduced, however the number of deflecting mirrors is two at a maximum, and there still is a problem of speeding up.
In order to solve the above problem, a light scanning device has been proposed that scans two different scanned faces by splitting one light beam from a light source into two light beams shifted in a sub-scanning direction by a light beam splitter, and scanning is performed by a deflector that coaxially rotates two polygon mirrors which are shifted at angles to each other and overlapped (see Japanese patent number 4568633).
In the technique disclosed in Japanese patent number 4568633, the two polygon mirrors of which a phase is shifted and overlapped are used as a scanner that scans different scanned faces by a light beam from a common light source, however the polygon mirrors of which the phase is shifted are not a general-purpose product, and therefore there is a possibility of an increase in cost. In addition, processing of the polygon mirrors is needed, and a face tangle error of each of the polygon mirrors is different and a surface accuracy is different, and therefore there is a possibility of a deterioration in image quality.
Additionally, a light scanning device that splits one light beam from a light source into two light beams having a main scanning direction by a light beam splitter, allows the two split light beams to enter a deflector having four reflecting faces with an aperture angle λ/2 via an incident mirror, and scans different scanned faces has been proposed (see Japanese patent application publication number 2008-257169).
In addition, as for the tandem-method image forming apparatus, when increasing the number of drums, there is a tendency for the image forming apparatus to increase in size, and therefore, including a light scanning device, downsizing has been demanded. For the purpose of downsizing, it is effective to overlap a plurality of optical paths of scanning light travelling toward each drum from a deflector.
For example, Japanese patent application publication number 2008-70599 discloses a light scanning device having a light source section that emits a plurality of light beams having different polarization directions, a single light deflector that deflects and scans the plurality of light beams emitted from the light source section in a main scanning direction, a scanning and focusing section that focuses each of the plurality of light beams on each of a plurality of corresponding scanned faces, and an optical element that is an optical path diverging section which is provided on an optical path between the light deflector and the scanned faces, and diverges optical paths of the plurality of light beams based on the polarization directions of the light beams, and on which a concave-convex configuration part having a pitch which is equivalent to, or less than or equal to a wavelength of the light beams is formed.
In the light scanning device disclosed in Japanese patent application publication number 2008-257169, in order to split the plurality of light beams toward the plurality of scanned faces, it is necessary to split the plurality of light beams so as to be at a distance from each other.
And in order to do so, there is a method such that the plurality of light beams are incident obliquely at a different angle to each other to a rotation axis of the deflector and the light beams reflected by the deflector are split by a splitting mirror to a predetermined distance. However, in the case of using this method, a light beam is incident obliquely on a scanning lens, and therefore, there is a possibility of deteriorating an optical characteristic such as bending of a scanning line and so on.
As another method, there is a method such that the plurality of light beams are incident vertically to the rotation axis of the deflector, and the plurality of light beams are separated on the deflector, and then the light beams are split by the splitting mirror. In the case of using this method, it is necessary to make the deflector larger in order that the plurality of light beams are separated and incident on the deflector, and therefore the cost of the deflector is increased.
Any of the above methods need a space to split the plurality of light beams, and therefore the light scanning device becomes large.
In the light scanning device disclosed in Japanese patent application publication number 2008-70599, the number of light sources are not reduced, and therefore it is not possible to reduce the cost of the light source section.